The most commonly employed photographic elements are those containing one or more radiation-sensitive silver halide emulsion layers. Their widespread use is attributable to the excellent quality images they are capable of producing and to their high speed, allowing them to be employed in hand-held cameras under a variety of lighting conditions.
Nevertheless, silver halide photographic elements have historically exhibited two significant limitations in terms of viewing the photographic image. First, imagewise exposure of the silver halide emulsion layer does not produce an immediately viewable photographic image. Exposure produces an invisible latent image in the silver halide emulsion. Processing of the latent image is required to produce a viewable image. Historically this has meant removing the photographic element from the camera and processing in one or more aqueous solutions to obtain a viewable image. Second, in most instances the first viewable image obtained is a negative image, and a second exposure through the negative image of an additional photographic element and processing thereof is required to produce a viewable positive of the image initially photographed. The first limitation can be overcome by employing image transfer techniques, and the second limitation can be overcome by employing direct-positive imaging, particularly direct reversal imaging.
a. Direct reversal imaging
Photographic elements which produce images having an optical density directly related to the radiation received on exposure are said to be negative-working. A positive photographic image can be formed by producing a negative photographic image and then forming a second photographic image which is a negative of the first negative--that is, a positive image. A direct-positive image is understood in photography to be a positive image that is formed without first forming a negative image. Positive dye images which are not direct-positive images are commonly produced in color photography by reversal processing in which a negative silver image is formed and a complementary positive dye image is then formed in the same photographic element. The term "direct reversal" has been applied to direct-positive photographic elements and processing which produces a positive dye image without forming a negative silver image. Direct-positive photography in general and direct reversal photography in particular are advantageous in providing a more straight-forward approach to obtaining positive photographic images.
A conventional approach to forming direct-positive images is to use photographic elements employing internal latent image-forming silver halide grains. After imagewise exposure, the silver halide grains are developed with a surface developer--that is, one which will leave the latent image sites within the silver halide grains substantially unrevealed. Simultaneously, either by uniform light exposure or by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of a surface latent image-forming photographic element. The internal latent image-forming silver halide grains which received actinic radiation during imagewise exposure develop under these conditions at a slow rate as compared to the internal latent image-forming silver halide grains not imagewise exposed. The result is a direct-positive silver image. In color photography, the oxidized developer that is produced during silver development is used to produce a corresponding positive, direct reversal dye image. Multicolor direct reversal photographic images have been extensively investigated in connection with image transfer photography.
It has been found advantageous to employ nucleating agents in preference to uniform light exposure in the process described above. The term "nucleating agent" is employed herein in its art-recognized usage to mean a fogging agent capable of permitting the selective development of internal latent image-forming silver halide grains which have not been imagewise exposed in preference to the development of silver halide grains having an internal latent image formed by imagewise exposure.
While nucleating agents have been long known to the photographic art, recent interest has focused on identifying nucleating agents that are effective in relatively low concentration levels and that can be incorporated directly into silver halide emulsions. Exemplary of known incorporated nucleating agents are those disclosed by Whitmore U.S. Pat. No. 3,227,552, Lincoln et al. U.S. Pat. No. 3,615,615, Kurtz et al. U.S. Pat. Nos. 3,719,494 and 3,734,738, Lincoln et al. U.S. Pat. No. 3,759,901, Leone et al. U.S. Pat. Nos. 4,030,925, 4,080,207, and 4,276,364, Adachi et al. U.S. Pat. No. 4,115,122, von Konig et al. U.S. Pat. No. 4,139,387, and U.K. Pat. Nos. 2,011,391 and 2,012,443. Nucleating agents particularly adapted for use in direct reversal photographic elements intended to be processed at lower pH levels are disclosed by Baralle et al. U.S. Pat. Nos. 4,306,016, 4,306,017, and 4,315,986.
Direct reversal emulsions useful with adsorbed nucleating agents include emulsions capable of forming latent image centers primarily in the interior of the silver halide grains as opposed to their surface--hereinafter also referred to as internal latent image-forming emulsions. Such emulsions can take the form of halide-conversion type emulsions, such as illustrated by Knott et al. U.S. Pat. No. 2,456,953 and Davey et al. U.S. Pat. No. 2,592,250, and core-shell emulsions, such as illustrated by Porter et al U.S. Pat. No. 3,206,313, Evans U.S. Pat. Nos. 3,761,276 and 3,923,513, and Atwell et al U.S. Pat. No. 4,035,185.
Direct reversal emulsions exhibit art-recognized disadvantages as compared to negative-working emulsions. Although Evans, cited above, has been able to increase photographic speeds by properly balancing internal and surface sensitivities, direct reversal emulsions have not achieved photographic speeds equal to the faster surface latent image-forming emulsions. Second, direct reversal emulsions are limited in their permissible exposure latitude. When exposure is extended rereversal occurs. That is, in areas receiving extended exposure a negative image is produced This is a significant limitation to in-camera use of direct reversal photographic elements, since candid photography does not always permit control of exposure conditions. For example, a very high contrast scene can lead to rereversal in some image areas.
A schematic illustration of rereversal is provided in FIG. 1, which plots density versus exposure. A characteristic curve l (stylized to exaggerate curve features for simplicity of discussion) is shown for a direct reversal emulsion. When the emulsion is coated as a layer on a support, exposed, and processed, a density is produced. The characteristic curve is the result of plotting various levels of exposure versus the corresponding density produced on processing. At exposures below level A underexposure occurs and a maximum density is obtained which does not vary as a function of exposure. At exposure levels between A and B useful direct reversal imaging can be achieved, since density varies inversely with exposure. If exposure occurs between the levels indicated by B and C, overexposure results. That is, density ceases to vary as a function of exposure in this range of exposures. If a subject to be photographed varies locally over a broad range of reflected light intensities, a photographic element containing the direct reversal emulsion can be simultaneously exposed in different areas at levels less than A and greater than B. The result may, however, still be aesthetically pleasing, although highlight and shadow detail of the subject are both lost. If it is attempted to increase exposure for this subject, however, to pick up shadow detail, the result can be to increase highlight exposure to levels above C. When this occurs, rereversal is encountered. That is, the areas overexposed beyond exposure level C appear as highly objectionable negative images, since density is now increasing directly with exposure. Useful exposure latitude can be increased by more widely separating exposure levels A and B, but this is objectionable to the extent that it reduces contrast below optimum levels for most subjects. Therefore reduction in rereversal is most profitably directed to increasing the separation between exposure levels B and C so that overexposed areas are less likely to produce negative images. (In actual practice the various segments of the characteristic curve tend to merge more smoothly than illustrated.)
b. Image transfer photography
Image transfer photography has made it possible to reduce the delay between imagewise exposure and obtaining a viewable image. Immediately after imagewise exposing the radiation-sensitive silver halide emulsion layer or layers, a processing solution can be brought into contact therewith. As silver halide development occurs, a black-and-white transferred silver image or a transferred dye image can be formed in a receiving layer for viewing. In this way, visual access to the photographic image can occur in minutes or even seconds.
Still, though measured in seconds, the delay in providing visual access remains an important limitation in silver halide image transfer photography. Subject opportunities can be fleeting, and the photographer needs as nearly an instantaneous visual verification of an acceptable photographic image as can be offerred.
Although image transfer has reduced the time required for image access in silver halide photography, this advantage has not been achieved without other sacrifices. One significant long term concern of image transfer photography relates to consumption of silver. Multicolor silver halide photographic elements which are conventionally processed and dye image transfer film units both employ relatively high silver coverages to obtain maximum photographic speed. Typically about 1000 milligrams per square meter of silver is required to form each of the blue, green, and red exposure records. In oonventionally processed multicolor photographic elements the image produced contains no silver and all of the silver present in the photographic element is, in theory, recoverable. On the other hand, in image transfer photography silver is seldom recovered, and in integral format image transfer film units all of the silver remains with the photographic film units forming the viewable image.
Another disadvantage, inherent in image transfer photography, is the reduction in image sharpness attributable to diffusion. As the image forming materials diffuse from the silver halide emulsion layer or an adjacent dye releasing layer, diffusion occurs both in the direction of the receiving layer and laterally, leading to image spreading and loss of sharpness. Sharpness can be improved by decreasing the length of the diffusion path to the receiving layer. This is controlled by the number and thickness of the layers to be traversed. Unfortunately, the minimum thickness of the silver halide emulsion layers is limited by the size of the silver halide grains and the weight ratio of gelatin to silver halide. Further, in multicolor image transfer film units employing three superimposed dye-providing layer units, intervening dye-providing layer units and separating interlayers must be penetrated by diffusing dyes migrating to the receiving layer.
Another consideration that arises in image transfer photography is image density variance as a function of temperature differences. Since subject opportunities are presented under a variety of temperature conditions and since the primary advantage of image transfer photography is ready image access, it follows that the ability of image transfer photographic elements to produce acceptable images at a variety of temperatures is also important. Image transfer photography is much different than conventional photography in this respect, since in the latter processing is rarely undertaken without control of temperature.
A number of imaging limitations are encountered in producing transferred images with dyes. For example, both the high silver coverages noted above and larger than stoichiometrically predicted amounts of dye-image-providing materials are required to obtain transferred dye images of acceptable maximum densities. To the extent that the efficiency of dye transfer declines from stoichiometrically predicted levels, more dye-image-providing materials must be incorporated in the photographic elements and the layer thicknesses must be increased to incorporate added amounts of these materials. Further, the rate of release of dyes for transfer can affect the time required to produce a viewable image. When the development reaction product is relied upon to preclude dye transfer, as in the case of many conventional positive-working dye-image-forms, the rate of silver halide development also limits the maximum rate at which image dye can become available for transfer, since too rapid release of image dye in relation to the rate of silver halide development can result in the loss of image discrimination. Improvements of any one or a combination of these characteristics can, of course, significantly improve dye image transfer.
Silver halide image transfer film units are generally well known in the art of photography and require no detailed description. Broad discussions of image transfer elements and processes (including process solutions) can be found in Chapter 12, "One Step Photography", Neblette's Handbook of Photography and Reprography Materials, Processes and Systems, 7th Ed. (1977), in Chapter 16, "Diffusion Transfer and Monobaths", T. H. James, The Theory of the Photographic Process, 4th Ed. (1977), and A. Rott and E. Weyde, Photographic Silver Halide Diffusion Processes, Focal Press, (1972). Patents relating to silver halide image transfer are collected in U.S. Patent and Trademark Office Class 430 RADIATION IMAGERY CHEMISTRY--PROCESS, COMPOSITION OR PRODUCT, subclasses 199 through 255.
Class 430, cited above, subclass 217 (Silver halide identified-grain, identified emulsion binder other than nominal gelatin, or identified sensitizer or identified desensitizer containing) contains a collection of patents directed to silver halide image transfer photography, many of which disclose specific silver halide grain structures.
c. Tabular silver halide grains
A great variety of regular and irregular grain shapes have been observed in silver halide photographic emulsions intended for imaging applications. Regular grains are often cubic or octahedral. Grain edges can exhibit rounding due to ripening effects, and in the presence of strong ripening agents, such as ammonia, the grains may even be spherical or near spherical thick platelets, as described, for example by Land U.S. Pat. No. 3,894,871 and Zelikman and Levi Making and Coating Photographic Emulsions, Focal Press, 1964, page 223. Rods and tabular grains in varied portions have been frequently observed mixed in among other grain shapes, particularly where the pAg (the negative logarithm of silver ion concentration) of the emulsions has been varied during precipitation, as occurs, for example in single-jet precipitations.
Tabular grains (those areally extended in two dimensions as compared to their thickness) have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. The term "substantially parallel" as used herein is intended to include surfaces that appear parallel on direct or indirect visual inspection at 10,000 times magnification. A discussion of tabular bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Silver Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide. Tabular silver bromide emulsions are discussed by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125. Sulfur sensitized tabular grain silver bromide emulsions having an average aspect ratio of from about 5 to 7:1 wherein the tabular grains account for greater than 50% of the projected area of the total grain population were incorporated in a direct X-ray radiographic product, No Screen X-Ray Code 5133 sold by Eastman Kodak Company from 1937 until the 1950's. Gutoff, "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographic Emulsions", Photographic Science and Engineering, Vol. 14, No. 4, July-August 1970, pp. 248-257, reports preparing silver bromide and silver bromoiodide emulsions of the type prepared by single-jet precipitations using a continuous precipitation apparatus.
Bogg, Lewis, and Maternaghan have recently published specific procedures for preparing emulsions in which a major proportion of the silver halide is present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 teaches forming silver halide crystals of tabular habit bounded by .sub.I 100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1. The tabular grains exhibit square and rectangular major surfaces characteristic of .sub.I 100} crystal faces. Lewis U.S. Pat. No. 4,067,739 teaches the preparation of silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals causing the seed crystals to increase in size by Ostwald ripening in the presence of a silver halide solvent, and completing grain growth without renucleation or Ostwald ripening while controlling pBr (the negative logarithm of bromide ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994 and 4,184,877, teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals which are at least 90 mole percent iodide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed; e.g., a grain consisting of silver bromiodide containing 40 mole percent iodide also contains 60 mole percent bromide.) Lewis and Maternaghan report increased covering power. Maternaghan states that the emulsions are useful in camera films, both black-and-white and color. Bogg specifically reports an upper limit on aspect ratios of 7:1, but, from the very low aspect ratios obtained by the examples, the 7:1 aspect ratio appears unrealistically high. It appears from repeating examples and viewing the photomicrographs published that the aspect ratios realized by Lewis and Maternaghan were also less than 7:1.
Maternaghan U.S. Pat. No. 4,184,878 (with which U.K. Pat. No. 1,570,581 and German OLS publications Nos. 2,905,655 and 2,921,077 are considered essentially cumulative) teaches the formation of direct-positive images by preparing a tabular grain emulsion essentially similarly as described by Maternaghan U.S. Pat. No. 4,184,877, but with the incorporation of an internal sensitizer and processing in a developer containing a nucleating agent.
Wilgus and Haefner U.S. Ser. No. 429,420, filed Sept. 30, 1982 and commonly assigned, titled HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS AND PROCESSES FOR THEIR PREPARATION, which is a continuation-in-part of U.S. Ser. No. 320,904, filed Nov. 12, 1981, now abandoned, discloses high aspect ratio silver bromoiodide emulsions and a process for their preparation.
Daubendiek and Strong U.S. Pat. No. 4,414,310, titled AN IMPROVED PROCESS FOR THE PREPARATION OF HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS, discloses an improvement on the processes of Maternaghan whereby high aspect ratio tabular grain silver bromoiodide emulsions can be prepared.
Solberg, Piggin, and Wilgus U.S. Ser. No. 431,913, filed Sept. 30, 1982 and commonly assigned, titled RADIATION-SENSITIVE SILVER BROMOIODIDE EMULSIONS, PHOTOGRAPHIC ELEMENTS, AND PROCESSES FOR THEIR USE, which is a continuation-in-part of U.S. Ser. No. 320,909, filed Nov. 12, 1981, now abandoned, discloses high aspect ratio tabular grain silver bromoiodide emulsions wherein a higher concentration of iodide is present in an annular region than in a central region of the tabular grains.
Wey U.S. Pat. No. 4,399,215, titled IMPROVED DOUBLE-JET PRECIPITATION PROCESSES AND PRODUCTS THEREOF, discloses a process of preparing tabular silver chloride grains which are substantially internally free of both silver bromide and silver iodide. The emulsions have an average aspect ratio of greater than 8:1.
Kofron et al U.S. Ser. No. 429,407, filed Sept. 30, 1982 and commonly assigned, titled SENSITIZED HIGH ASPECT RATIO SILVER HALIDE EMULSIONS AND PHOTOGRAPHIC ELEMENTS, which is a continuation-in-part of U.S. Ser. No. 320,905, filed Nov. 12, 1981, now abandoned, discloses chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions and photographic elements incorporating these emulsions.
Mignot U.S. Pat. No. 4,386,156, titled SILVER BROMIDE EMULSIONS OF NARROW GRAIN SIZE DISTRIBUTION AND PROCESSES FOR THEIR PREPARATION, discloses high aspect ratio tabular grain silver bromide emulsions wherein the tabular grains are square or rectangular.
Dickerson U.S. Pat. No. 4,414,304, titled FOREHARDENED PHOTOGRAPHIC ELEMENTS AND PROCESSES FOR THEIR USE, discloses producing silver images of high covering power by employing photographic elements containing forehardened high aspect ratio tabular grain silver halide emulsions.
Abbott and Jones U.S. Ser. No. 430,222, filed Sept. 30, 1982 and commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED CROSSOVER, which is a continuation-in-part of U.S. Ser. No. 320,907, filed Nov. 12, 1981, now abandoned, discloses the use of high aspect ratio tabular grain silver halide emulsions in radiographic elements coated on both major surfaces of a radiation transmitting support to control crossover.
Jones and Hill U.S. Ser. No. 553,911, filed Nov. 21, 1983 and commonly assigned, titled PHOTOGRAPHIC IMAGE TRANSFER FILM UNIT, which is a continuation-in-part of U.S. Ser. No. 430,092 filed Sept. 30, 1982, which is in turn a continuation-in-part of U.S. Ser. No. 320,911, filed Nov. 12, 1981, now abandoned, discloses image transfer film units containing tabular grain silver halide emulsions.
Hoyen U.S. Pat. No. 4,395,478, titled DIRECT-POSITIVE CORE-SHELL EMULSIONS AND PHOTOGRAPHIC ELEMENTS AND PROCESSES FOR THEIR USE, discloses the use of divalent or trivalent metal ion dopants in the shell of core-shell emulsions to reduce rereversal.